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Old 26th May 2004, 02:37 PM   #1
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Default short circuit protection

what is a simple way to provide short citcuit protection ?
i have an sony amp that has and it works nicely as the music cuts out for like 20 secs of shorted
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Old 26th May 2004, 03:16 PM   #2
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Can you explain this.

Do you short the cables after 20 seconds or do you have music in 20 seconds before some protection goes on?
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Old 26th May 2004, 03:21 PM   #3
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Third interpretation:

Amp goes quiet for 20 seconds after short on output.

Rune
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Old 26th May 2004, 04:29 PM   #4
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Default Re: short circuit protection

Quote:
Originally posted by roofingboom
what is a simple way to provide short citcuit protection ?
i have an sony amp that has and it works nicely as the music cuts out for like 20 secs of shorted
I don't know what you consider "simple" (simple is relative), but typically current sensing is done with a low Ohm resistor (milli-ohms) in series with a voltage. The voltage drop across the resistor is sensed across a comparator with a reference. When the current (read as voltage drop across the sense resistor) hits the comparator threshold, the comparator turns on. The comprator then removes power in some way, typically by opening a switch, such as a FET. You can build in a delay so that the unit can attempt to turn back on after some period of time, in this case apparently 20 seconds.

You always want some hysterisis on a comparator, or a short circuit condition would cause oscillation as the comparator ties to turn off and back on instantly.

Are you trying to design your own short circuit protection?

Hope this helps.

Cheers,

bg


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Old 27th May 2004, 08:48 AM   #5
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yes i am trying to make my own short circuit protection
i had a problem yesterday i was playing music into an unrated speaker and it got too hot causing it to short internally causing my amp to short and poof the power transistors went up in flames
i am wondering what i can do to prevent this
maybe a starting schematic would be nice
i like your explanantion beer guy are there some links to some start schematics possibly?

nate
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Old 27th May 2004, 09:01 AM   #6
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is a dc protection circuit the same thing as short circuit circuit? same thing different name?
the protection on my sony amp is really nice because it cuts out and trys to turn back on in like 20 secconds
this would save me tons of trouble since i just fried some output transistors due to speaker failure
any information schematics pointers for me would be very useful
thanks
nate
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Old 27th May 2004, 10:39 AM   #7
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Quote:
is a dc protection circuit the same thing as short circuit circuit?
Not really. When a short exists there is no dc across the speaker so a dc detector won't detect anything. You usually get dc when a power transistor fails and the dc ends up frying your speaker. In the case of a short circuit it is your speaker that fries your transistors.

I often use both a "fast" current limiter at the power-transistors themselves and a dc-detector that disconnects the speakers if the dc level exceeds +/- 0.7V. My primary concern is the speaker rather than the amp (new speakers are much more expensive than new transistors).

In theory, what you need to protect the amp is to measure the resistance of the speaker and then disconnect the speaker if it is too low. This can be done with many levels of sophistication. One question to ask yourself is how low can the speaker resistance be before the protection kicks in?

One method might be to put a current sensing resistor in series with the speaker, say between speaker and gnd. Then measure the ratio of voltage at amp output to that at the sensing resistor. If the ratio is too low then cut the speaker off using a relay.

Eg: sensing resitor is 0.5-ohms. Speaker resistance is 8-ohms. Both are in series so if the voltage between amp out and gnd is Vamp, then the voltage across the sense resistor will be Vr = Vamp x 0.5 / (8 + 0.5) = 0.06Vamp. Now, if the speaker is shorted the ratio will be 1. If the speaker is 1-ohm the ratio will be 0.3. You can make an op-amp circuit to measure the ratio and trigger a relay if it is too high.

Another thing you need is to make sure the transistors don't suffer damage while the protection circuit is turning off the relay. There will be a millisecond delay due to the mechanics. You need a fast circuit near each output transistor to limit the peak current to within the specs of the transistors during this delay. This is easy for MOSFETs (often just a zener across gate and source) but more complicated for bipolars (transistor and low-pass filter).
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Old 28th May 2004, 12:18 PM   #8
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Quote:
Originally posted by roofingboom
yes i am trying to make my own short circuit protection
i had a problem yesterday i was playing music into an unrated speaker and it got too hot causing it to short internally causing my amp to short and poof the power transistors went up in flames
i am wondering what i can do to prevent this
maybe a starting schematic would be nice
i like your explanantion beer guy are there some links to some start schematics possibly?

nate
Nate,


I'm in the middle of moving both my lab and office to another building, so I'm a bit swamped at the moment. I'll try to post a basic schematic after the holiday break. You can probably get started by Googling "current Sense".

Cheers,

bg

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Old 28th May 2004, 02:22 PM   #9
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take a look at the protection circuit in the Leach Amp. The output current is sensed at the emitter resistors. Too much voltage and the protection transitors kick in and remove the drive signal. It seems to work well, although I don't think I get mine anywhere near the current limit.

If you don't like the idea of having the protection circuit potentially messing with the signal, use the same scheme to drive a relay with some sort of hold open delay. The transistor could also be used to feed some DC into your dc protection circuit and trip that.
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Old 28th May 2004, 02:43 PM   #10
thanh is offline thanh  Viet Nam
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i use slone's short circuit protection! it is quite bad.There were 5 pair of power transistors go to the rubbish
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